With the wafer size being enlarged to 300 mm currently and even to 450 mm in the near future, the chips at the wafer edge have more influence upon the total output. Therefore, it is quite important for measuring the thickness of the film on the edge of the wafer (i.e. the film thickness of the edge of the film of the wafer) during a chemical mechanical polishing process.
Thickness and surface morphology of a copper film on the wafer are generally measured by using an eddy current transducer before or after the chemical mechanical polishing process. Generally a radius of an eddy current transducer probe is at least 6-8 mm. FIG. 1 shows a film thickness curve measured by the eddy current transducer in the prior art, in which curve 1 is a real film thickness curve, and curve 2 is a measured curve obtained by using the eddy current transducer. FIG. 2 illustrates a wafer measuring process in the prior art. As shown in FIG. 2, eddy current transducers 6 and 7 are arranged with opposing to each other, and a wafer 8 such as a silicon wafer gradually passes through the space between the eddy current transducers 6 and 7 so that the film thickness of the wafer may be detected by the eddy current transducers 6 and 7. The film thicknesses measured at positions a, b, c in FIG. 2 are corresponding to the points 5, 4, 3 on the film thickness curve of FIG. 1. In theory, the eddy current transducers receive a measuring signal so as to obtain the film thickness only when the wafer arrives at position b. However, in practical measuring processes, the eddy current transducers should have already received the measuring signal at the position a as shown in FIG. 2 before the wafer has approached the eddy current transducers. Therefore, it is difficult to accurately measure the film thickness of the film in an annular edge region within 15 mm from the outer periphery of the wafer, thus causing inaccurate film thickness measurement of the wafer.